MEGAHERTZ FAR-INFRARED FREQUENCY-SHIFTING VIA MULTIPLE DIFFRACTION BY A ROTATING GRATING

Frequency shifts in the megahertz region, as required under certain conditions in far-infrared diagnostics of magnetically confined fusion plasmas, can be achieved via multiple diffraction by a cylindrical diffraction grating rotating at moderate speed. We present a compact modular concept to correct the beam distortion at the grating. The concept is based on two elliptical mirrors and a sequence of (n-1) identical concave mirrors for n diffractions. Design parameters are derived for lambda = 119 mum (H2O laser), lambda = 195 mum (DCN laser), and lambda = 337 mum (HCN laser). The concept was successfully realized for lambda = 1 19 mum using four diffractions. A frequency shift of 3 MHz was achieved with a grating driven by an electric micromotor. The quality of the beam exiting from the grating was excellent. The total efficiency after four diffractions was measured as 64%, corresponding to an overall efficiency (i.e., mirror and other losses included) of 0.64(1/4) = 89% per diffraction.